Pulse oximetry is a diagnostic method that measures the proportion of oxygen-carrying molecules in the blood (called hemoglobin) that are actually carrying oxygen. This is known as oxygen saturation or SpO2. A conventional pulse oximeter sensor shines two light beams of different wavelengths (e.g., red and infrared) through the blood that is circulating in the small blood vessels of an extremity (e.g., the finger or ear), and then detects the amount of light that is able to pass through the extremity. Hemoglobin carrying oxygen (red blood) absorbs more infrared light and allows more red light to pass than hemoglobin without oxygen (blue blood) which allows more infrared light to pass. The oxygen saturation is expressed as a percentage of hemoglobin that has oxygen attached to it. One hundred percent oxygen saturation is attained when all the hemoglobin in the blood is completely saturated with oxygen.
Oxygen is the source of life for all cells in the body. Oxygen is carried from the lungs to our cells by the blood. A short interruption in the supply of oxygen will kill cells, and a slightly longer interruption can kill the whole body. Hemoglobin in the blood is normally almost fully saturated with oxygen (SpO2=95-100%). A decrease below this level indicates that either the amount of oxygen being delivered throughout the body is reduced or the amount being used by the body has increased. Pulse oximetry may be used to detect reduced levels of oxygen in the blood before the clinical sign of oxygen deprivation (skin turns blue) can be seen. This early detection of a reduced level of oxygen allows early rescue before levels of oxygen drop to critical levels. Pulse oximetry has contributed significantly to reducing the risk of death associated with anesthesia.
Since its introduction into the operating room in the 1980s, pulse oximetry has been routinely used to monitor patients who are under anesthesia during surgery. Its use has spread throughout the hospital so that any patient with unstable oxygen levels may be monitored, for example, in the intensive care unit, the emergency department or on the ward. Pulse oximetry can also be used at home by patients with lung disease.
Pulse oximetry has the potential to act as diagnostic device that can identify early signs of lung disease or as indicators of the severity of diseases that affect the whole body. Typically, SpO2 decreases as these diseases progressively worsen, when the lungs begin to fail and the body's use of oxygen increases. A decrease in blood oxygen saturation (SpO2), resulting from impeded gas exchange in the lungs, is a strong predictor of critical illness in pneumonia and other infectious or inflammatory diseases. Indeed, pneumonia diagnosis based on a low SpO2 can differentiate severe from mild respiratory tract infections, such as the common cold.
Prior art diagnostic pulse oximeters are typically bulky (e.g. they may weigh over 2 kg with a large footprint) and expensive custom built devices that are not available in many locations where they might be clinically useful. The World Health Organization (WHO) estimates a shortage of 90,000-150,000 oximeters in hospitals worldwide for anesthesia.
In many prior art oximeters electrical waveform signals used to generate the oximeter sensor light beams are of a substantially rectangular nature. Rectangular waves contain a high sub-band content of harmonics of the fundamental frequency and this introduces artifacts in the measured signal in the form of noise aliasing. In particular, harmonics of the line frequency are difficult to eliminate. In addition, many prior art oximeters make use of time division multiplexing to analyze detected light signals which can introduce challenges to distinguish between light from the sensor emitters and spurious ambient light.
Further, in many prior art oximeters, alarms are based on fixed thresholds that typically do not account for patient demographics and intra-patient variability. Often the generated alarms are unreliable, and clinicians tend to consider them to be a distraction.
The inventors have determined a need for improved methods and systems for conducting pulse oximetry. The inventors have determined a particular need for methods and systems for conducting pulse oximetry which are low cost, accurate, and robust.